1. Field of the Invention
This invention relates to optical waveguide couplers and more particularly to switchable optical couplers.
2. Description of the Prior Art
Electro-optic materials such as LiTaO.sub.3 and LiNbO.sub.3 have been employed to provide induced optical waveguides such as those disclosed in U.S. Pat. Nos. 3,695,745 and 3,795,433. These optical waveguides are formed by the realization of a refractive index change caused by applying a voltage to electrodes positioned on the electro-optic material (substrate) to increase the refractive index between the electrodes relative to the refractive index external thereto. At times the voltage required approaches the breakdown voltage of the substrate and a tenuous operating safety factor exists. An alternative configuration uses pairs of electrodes positioned on either side of the desired channel to which the voltages applied decrease the refractive index between the electrodes relative to that within the desired guiding channel. This configuration, however, may also require voltages near the breakdown voltage of the substrate.
Switchable couplers existing in the prior art with the exception of the coupler described by Soref et al in Applied Physics Letters, Volume 28, No. 12 on page 716 all operate with single mode radiation only, such as the coupler described by Burns et al in Applied Physics, volume 15, on page 1053. These single mode couplers are interferometric in nature and and must be manufactured to strict tolerances. Soref et al disclose a coupler wherein properly positioned electrodes are deposited on a substrate with a gap therebetween to establish a main channel and a branch channel. A voltage is applied to the main channel electrodes across the thickness of the substrate in a manner to induce a refractive index increase to form a light waveguide between the main channel electrodes. With a reverse voltage or no voltage applied to the branch electrodes, light energy theoretically does not couple to the branch channel. When a voltage that is of equal amplitude and of the same polarity to that coupled to the main channel electrodes is applied to the branch channel electrodes, the light energy propagating in the main channel is caused to divide between the two channels. To effectuate the desired optical waveguides and switching, relatively high voltages must be utilized and switched. This limits the rapidity with which optical energy may be transferred from the main channel to the branch channel and may also exhibit erratic operation if the required voltages approach the breakdown voltage of the substrate.